gpio: sx150x: use the new open drain callback

One variant of the SX150X GPIO chip supports setting the pins in
open drain mode. This is currently available to set from platform
data, but completely unused in the kernel.

Activate the new .set_single_ended() callback so users can set
this up from e.g. device tree or board files using the new
GPIO descriptors.

As part of this, delete the platform data open drain setting
method.

Cc: Wei Chen <Wei.Chen@csr.com>
Cc: Peter Rosin <peda@axentia.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>